1. Field of the Invention
The present invention relates to a power semiconductor device, and particularly relates to a power semiconductor device in which an IGBT and a MOSFET operated in parallel are used as switching devices.
2. Description of the Background Art
In a switching device such as an IGBT (insulated gate bipolar transistor), a configuration in which a MOSFET (MOS field effect transistor) is connected in parallel with the IGBT for the purpose of reducing a switching loss has conventionally been studied.
For example, Japanese Patent Application Laid-Open No. 4-354156 (1992) discloses, in FIG. 5, a configuration in which gates of an IGBT and a MOSFET connected in parallel with each other are commonly connected and they are driven by a common gate drive circuit.
In a case where such a configuration is adopted, by making use of a difference between the threshold voltages of the IGBT and the MOSFET, turn-off characteristics of the MOSFET can be reflected in transient characteristics at a time of turning off, so that turn-off characteristics of the IGBT that causes a large turn-off loss are absorbed, to thereby reduce a switching loss.
In the configuration disclosed in Japanese Patent Application Laid-Open No. 4-354156 (1992) mentioned above, an ON threshold voltage of the IGBT is set higher than an ON threshold voltage of the MOSFET, and therefore a total current flows in the MOSFET without fail in a transient state at a switching time. To deal with it, the current rating of the MOSFET has to be increased, which makes it difficult to reduce the chip size of the MOSFET. Thus, there has been a problem that it is difficult to downsize the whole of a device.